perf(desktop): speed up local-server install — uv on Windows, background Playwright/Chromium with mirror fallback

[Luhaozhu]

What

Speeds up the desktop local-server bootstrap on fresh machines, where installing every dependency from official sources could take 10+ minutes on slow networks.

Windows: install Python dependencies with uv

• install-local-server.ps1 now bootstraps uv into the venv and routes all dependency installs (server, mem0, script/document tools, editable install) through uv pip install — parallel downloads plus a much faster resolver, matching what the macOS bootstrap already does.
• If uv can't be installed or an install step fails, each step falls back to the original pip install path automatically, so reliability is unchanged.

Both platforms: Playwright/Chromium moves to a background install with mirror fallback

• pptxgenjs + playwright + the ~130 MB Chromium download no longer block the install: the core service now finishes installing and starts first, while the optional document tools install in a detached background task (log: node-tools-install.log, with a lock file to prevent duplicate runs).
• The background task probes registry.npmjs.org; when it is slow or unreachable it switches to the npmmirror registry and PLAYWRIGHT_DOWNLOAD_HOST mirror for the Chromium binary. A failed official-host Chromium download is retried once via the mirror.
• Failure behaviour is unchanged: Word/Excel/PPT generation keeps working, advanced PDF rendering uses its existing fallback until Chromium is available.

Testing

• npm run test:scripts in desktop/ — 16/16 pass
• bash -n on the modified shell script